Twitter's co-founder Evan Williams is stepping down from the company's board of directors, he said Friday.

"It's been an incredible 13 years, and I'm proud of what Twitter has accomplished during my time with the company," Williams said in a statement. "I will continue rooting for the team as I focus my time on other projects."

Williams will leave at the end of the month, according to a filing with the US Securities and Exchange Commission. When asked to comment, a Twitter representative pointed to a tweet linking to the regulatory filing on Williams' departure.

"I'm very lucky to have served on the Twitter board for 12 years (ever since there was a board)," Williams tweeted Friday. "It's been overwhelmingly interesting, educational -- and, at times, challenging."

I'm very lucky to have served on the @Twitter board for 12 years (ever since there was a board). It's been overwhelmingly interesting, educational—and, at times, challenging.